1. Field of the Invention
The present invention relates to a system for transferring wideband sound signals and more particularly to a system wherein a lower frequency range is transmitted directly and a higher frequency range is divided into partial bands of which only amplitude information is transmitted on pilot signals.
2. Description of the Prior Art
My U.S. Pat. Nos. 3,875,341 and 3,894,190 describe systems for transferring wideband sound signals wherein, at the input end of the system, the sound signal is divided into a lower and a higher frequency range. The partial signal of the lower frequency range is transferred directly and instead of the partial signal of the higher frequency range being transferred, the amplitude information of the partial frequency ranges obtained by splitting up said higher frequency range by means of bandpass filters is transferred. For reproducing the total signal, the amplitude information of the partial frequency ranges serves as a modulating signal for equivalent signals lying approximately in the middle of the individual partial frequency ranges to produce a synthetic signal, and said synthetic sound signals of the partial frequency ranges of the higher frequency range are added to the directly transferred partial signal of the lower frequency range.
During the practical operation of such a system, it has become apparent that at the reproducing end, the restoration of the sound signals of certain tone sources, which include the human voice in particular, does not sufficiently meet the requirements placed on high-quality reproduction.
Subsequent investigations have shown that this phenomenon is connected with the peculiar characteristic of these tone sources, such as the human voice, for the latter is a tone source whose sound signals have a continuous spectrum at least in the upper frequency range.
Bandwidth compression of speech signals has been proposed in U.S. Pat. Nos. 3,030,450 and 3,431,362 and these patents propose the use of noise signals to create a more realistic reproduction of speech signals. These systems are satisfactory for telephone speech reproduction but do not provide hi fidelity reproduction.
I have already proposed, in my previously mentioned patents, providing at the input end of the system, i.e., e.g., at the transmitting end, a second pick-up channel to which those sound signals are applied which have a predominantly continuous spectrum in the upper frequency range. In this channel, a split-up into an upper and a lower frequency range takes place, with the partial signal of the lower frequency range being added to that of the first pick-up channel and the partial signal of the upper frequency range being applied to a single rectifier circuit for the formation of an amplitude information. At the reproducing end, this amplitude information serves to modulate a noise voltage which has approximately the same frequency spectrum as the upper frequency range of the second pick-up channel.